This invention relates to a method of maintaining acid catalyst sites of a silicoaluminophosphate (SAPO) molecular sieve. The molecular sieve can be used to make an olefin product by contacting the activated molecular sieve with an oxygenate feedstock. Preferred oxygenate feedstocks are alcohols and ethers, and the olefin product desirably contains high concentrations of ethylene and propylene. The invention also relates to a SAPO molecular sieve which exhibits at least one peak in a specific IR spectrum.
Silicoaluminophosphates (SAPOs) have been used as absorbents and catalysts. As catalysts, SAPOs have been used in processes such as fluid catalytic cracking, hydrocracking, isomerization, oligomerization, the conversion of alcohols or ethers, and the alkylation of aromatics. To be useful as a catalysts, SAPO molecular sieves have to be activated. Methods of activating SAPOs are known.
U.S. Pat. No. 4,681,864 to Edwards et al. discuss the use of SAPO-37 molecular sieve as a commercial cracking catalyst. It is disclosed that activated SAPO-37 molecular sieve has poor stability. However, stability can be improved by using a particular activation process. According to the process, organic template is removed from the core structure of the sieve just prior to contacting with feed to be cracked. The process calls for subjecting the sieve to a temperature of 400-800xc2x0 C. within the catalytic cracking unit.
U.S. Pat. No. 5,185,310 to Degnan et al. discloses another method of activating silicoaluminophosphate molecular sieve compositions. The method calls for contacting a crystalline silicoaluminophosphate with gel alumina and water, and thereafter heating the mixture to at least 425xc2x0 C. The heating process is first carried out in the presence of an oxygen depleted gas, and then in the presence of an oxidizing gas. The object of the heating process is to enhance the acid activity of the catalyst. The acid activity is enhanced as a result of the intimate contact between the alumina and the sieve.
Briend et al., J. Phys. Chem. 1995, 99, 8270-8276, teach that SAPO-34 loses its crystallinity when the template has been removed from the sieve and the de-templated, activated sieve has been exposed to air. Data is presented, however, which suggest that over at least the short term, crystallinity loss is reversible. Even over a period of a couple years, the data suggest that crystallinity loss is reversible when certain templates are used.
As seen from the disclosure herein, we have found that SAPO molecular sieves will exhibit a shortened catalytic life when the catalytic sites are activated and exposed to a moisture-containing environment. This loss of catalytic life is likely not reversible, and can occur over a very short period of time. In essence, this loss of catalytic life is due to a loss in the number of acid catalytic sites. It is desirable, therefore, to obtain a SAPO molecular sieve that can be activated with little to no loss in acid catalyst sites, and exhibit a relatively long catalytic life. Such a molecular sieve would be highly desirable for use in a wide variety of catalytic compositions, which could be used in a variety of catalytic processes.
In order to overcome at least one of the many problems inherent in the prior art, the invention provides a method of maintaining acid catalyst sites of a silicoaluminophosphate (SAPO) molecular sieve. SAPO molecular sieves are compositions which act as catalysts in converting various hydrocarbon feeds to products. The compositions are manufactured by reacting a mixture of several chemical components. One of the components used in the reaction process is a template, although more than one template can be used. The templates are used to form channels or tunnel like structures (also called microporous structure) within the SAPO composition. To activate the SAPO composition so that it can be used as a catalyst, the template must be removed. These leaves behind an open microporous structure in which feedstock can enter, contact the catalytic sites, and the resulting product can exit. The manner in which the template is removed will affect the acid catalyst sites that are within the microporous structure.
In order to preserve catalytic activity, i.e., maintain acid catalyst sites, this invention provides a method which comprises providing a template-containing silicoaluminophosphate molecular sieve and heating the template-containing silicoaluminophosphate molecular sieve in an oxygen depleted environment under conditions effective to provide an integrated catalyst life that is greater than that obtained in a non-oxygen depleted environment. The heated molecular sieve exhibits at least one peak in the infrared region in a range of from 3630 cmxe2x88x921 to 3580 cmxe2x88x921. Preferably, the heated molecular sieve exhibits a combined peak area in the 3630 cmxe2x88x921 to 3580 cmxe2x88x921 range of at least 10% of a total peak area of all peaks in an infrared region between 4000 cmxe2x88x921 to 3400 cmxe2x88x921.
In a preferred embodiment, the activated molecular sieve can be used to make an olefin product from an oxygenate feedstock. This method comprises providing an activated molecular sieve exhibiting a total peak area in an infrared region from 3630 cmxe2x88x921 to 3580 cmxe2x88x921 that is at least 10% of a total peak area in an infrared region from 4000 cmxe2x88x921 to 3400 cmxe2x88x921, and contacting the activated molecular sieve with the oxygenate feedstock under conditions effective to convert the oxygenate feedstock to an olefin product. The olefin product made according to this method is also included as part of the invention. In addition, the invention includes a polyolefin made by contacting the olefin product with a polyolefin-forming catalyst under conditions effective to convert the olefin to polyolefin.
The molecular sieve is preferably selected from the group consisting of SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, and SAPO-56, metal containing forms thereof, and mixtures thereof. More preferably, the molecular sieve is selected from the group consisting of SAPO-18, SAPO-34, SAPO-35, SAPO-44, SAPO-47, metal containing forms thereof and mixtures thereof.
In another preferred embodiment the heated molecular sieve exhibits an integrated catalyst life of at least 4 grams of methanol converted per gram of molecular sieve. It is also preferred that the molecular sieve have a pore size of less than 5 angstroms.
In the method, it is preferred that heating be carried out in an oxygen depleted environment. Preferably, the oxygen depleted environment is substantially free of O2. More preferably, the oxygen depleted environment has an oxygen partial pressure of less than about 1.0 psia, more preferably less than about 0.2 psia.
The molecular sieve is heated under conditions sufficient to remove substantially all of the template from the molecular sieve. Preferably, the molecular sieve is heated to a temperature of 200 to 800xc2x0 C. The molecular sieve is preferably heated substantially in the absence of oxygen.
In yet another preferred embodiment, the template is selected from the group consisting of a tetraethyl ammonium salt, cyclopentylamine, aminomethyl cyclohexane, piperidine, triethylamine, cyclohexylamine, tri-ethyl hydroxyethylamine, morpholine, dipropylamine, pyridine, isopropylamine and mixtures thereof.